Plants require certain essential nutrients for normal functioning and growth. Nutrient levels outside the amount required for normal functioning and growth may cause overall crop growth and health to decline due to either a deficiency or a toxicity. Nutrient deficiency occurs when an essential nutrient is not available in sufficient quantity to meet the requirements of a growing plant. Toxicity occurs when a nutrient is in excess of plant needs and decreases plant growth or quality.
Plant nutrients are divided into two categories: macronutrients, which are consumed in larger quantities and may be present in plant tissue in quantities from about 0.2% to about 4.0% by dry matter weight; and micronutrients, which are consumed in smaller quantities and may range from about 5 parts per million (ppm) to about 200 ppm or less than about 0.2% dry weight. Macronutrients include carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, magnesium, and silicon. Micronutrients include iron, molybdenum, boron, copper, manganese, sodium, zinc, nickel, chlorine, selenium, vanadium and cobalt.
There are three fundamental ways plants uptake nutrients through the root: (1) simple diffusion, where a nonpolar molecule, such as, for example, O2, CO2, and NH3 that follow a concentration gradient, can passively move through the lipid bilayer membrane without the use of transport proteins; (2) facilitated diffusion, where the rapid movement of solutes or ions following a concentration gradient is facilitated by transport proteins; and (3) active transport, in which the active transport of ions or molecules against a concentration gradient requires an energy source, usually ATP, to pump the ions or molecules through the membrane.
However, not all plant nutrients are equally mobile. For example, boron is generally considered to be phloem immobile or to have only limited phloem mobility in higher plants. The mobility of boron from outside the plant cell to inside the plant cell typically involves mediation by a boron polyol complex. Polyol compounds, however, may not be present sufficiently or may be totally absent in higher plants. Thus, in most commercial field crops and horticultural crops (which lack polyol), boron's mobility is restricted and boron fertilization is limited. Thus, there is a need for a more mobile and efficient nutrient fertilization method in plants.